# Standard library header <memory>

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< cpp‎ | header

This header is part of the dynamic memory management library.

### Includes
<compare> (C++20) | Three-way comparison operator support
### Classes
##### Pointer traits
pointer_traits (C++11) | provides information about pointer-like types (class template)
##### Garbage collector support
pointer_safety (C++11)(removed in C++23) | lists pointer safety models (enum)
##### Allocators
allocator | the default allocator (class template)
allocator_traits (C++11) | provides information about allocator types (class template)
allocation_result (C++23) | records the address and the actual size of storage allocated by `allocate_at_least` (class template)
uses_allocator (C++11) | checks if the specified type supports uses-allocator construction (class template)
##### Uninitialized storage
raw_storage_iterator (deprecated in C++17)(removed in C++20) | an iterator that allows standard algorithms to store results in uninitialized memory (class template)
##### Smart pointers
unique_ptr (C++11) | smart pointer with unique object ownership semantics (class template)
shared_ptr (C++11) | smart pointer with shared object ownership semantics (class template)
weak_ptr (C++11) | weak reference to an object managed by std::shared_ptr (class template)
auto_ptr (deprecated in C++11)(removed in C++17) | smart pointer with strict object ownership semantics (class template)
##### Helper classes
std::atomic<std::shared_ptr> (C++20) | atomic shared pointer (class template specialization)
std::atomic<std::weak_ptr> (C++20) | atomic weak pointer (class template specialization)
owner_less (C++11) | provides mixed-type owner-based ordering of shared and weak pointers (class template)
owner_hash (C++26) | provides owner-based hashing for shared and weak pointers (class)
owner_equal (C++26) | provides mixed-type owner-based equal comparisons of shared and weak pointers (class)
enable_shared_from_this (C++11) | allows an object to create a `shared_ptr` referring to itself (class template)
bad_weak_ptr (C++11) | exception thrown when accessing a `weak_ptr` which refers to already destroyed object (class)
default_delete (C++11) | default deleter for unique_ptr (class template)
std::hash<std::unique_ptr> (C++11) | hash support for `std::unique_ptr` (class template specialization)
std::hash<std::shared_ptr> (C++11) | hash support for `std::shared_ptr` (class template specialization)
##### Smart pointer adaptors
out_ptr_t (C++23) | interoperates with foreign pointer setters and resets a smart pointer on destruction (class template)
inout_ptr_t (C++23) | interoperates with foreign pointer setters, obtains the initial pointer value from a smart pointer, and resets it on destruction (class template)
##### Forward declarations
Defined in header `<functional>`
hash (C++11) | hash function object (class template)
Defined in header `<atomic>`
atomic (C++11) | atomic class template and specializations for bool, integral, floating-point,(since C++20) and pointer types (class template)
### Tags
allocator_argallocator_arg_t (C++11) | a tag used to select allocator-aware constructors (tag)
### Functions
##### Uses-allocator construction
uses_allocator_construction_args (C++20) | prepares the argument list matching the flavor of uses-allocator construction required by the given type (function template)
make_obj_using_allocator (C++20) | creates an object of the given type by means of uses-allocator construction (function template)
uninitialized_construct_using_allocator (C++20) | creates an object of the given type at specified memory location by means of uses-allocator construction (function template)
##### Miscellaneous
to_address (C++20) | obtains a raw pointer from a pointer-like type (function template)
addressof (C++11) | obtains actual address of an object, even if the `&` operator is overloaded (function template)
align (C++11) | aligns a pointer in a buffer (function)
assume_aligned (C++20) | informs the compiler that a pointer is aligned (function template)
is_sufficiently_aligned (C++26) | checks whether the pointer points to an object whose alignment has at least the given value (function template)
##### Explicit lifetime management
start_lifetime_asstart_lifetime_as_array (C++23) | implicitly creates objects in given storage with the object representation reused (function template)
##### Garbage collector support
declare_reachable (C++11)(removed in C++23) | declares that an object can not be recycled (function)
undeclare_reachable (C++11)(removed in C++23) | declares that an object can be recycled (function template)
declare_no_pointers (C++11)(removed in C++23) | declares that a memory area does not contain traceable pointers (function)
undeclare_no_pointers (C++11)(removed in C++23) | cancels the effect of std::declare_no_pointers (function)
get_pointer_safety (C++11)(removed in C++23) | returns the current pointer safety model (function)
##### Uninitialized storage
uninitialized_copy | copies a range of objects to an uninitialized area of memory (function template)
uninitialized_copy_n (C++11) | copies a number of objects to an uninitialized area of memory (function template)
uninitialized_fill | copies an object to an uninitialized area of memory, defined by a range (function template)
uninitialized_fill_n | copies an object to an uninitialized area of memory, defined by a start and a count (function template)
uninitialized_move (C++17) | moves a range of objects to an uninitialized area of memory (function template)
uninitialized_move_n (C++17) | moves a number of objects to an uninitialized area of memory (function template)
uninitialized_default_construct (C++17) | constructs objects by default-initialization in an uninitialized area of memory, defined by a range (function template)
uninitialized_default_construct_n (C++17) | constructs objects by default-initialization in an uninitialized area of memory, defined by a start and a count (function template)
uninitialized_value_construct (C++17) | constructs objects by value-initialization in an uninitialized area of memory, defined by a range (function template)
uninitialized_value_construct_n (C++17) | constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (function template)
construct_at (C++20) | creates an object at a given address (function template)
destroy_at (C++17) | destroys an object at a given address (function template)
destroy (C++17) | destroys a range of objects (function template)
destroy_n (C++17) | destroys a number of objects in a range (function template)
get_temporary_buffer (deprecated in C++17)(removed in C++20) | obtains uninitialized storage (function template)
return_temporary_buffer (deprecated in C++17)(removed in C++20) | frees uninitialized storage (function template)
##### Smart pointer non-member operations
make_uniquemake_unique_for_overwrite (C++14)(C++20) | creates a unique pointer that manages a new object (function template)
operator==operator!=operator<operator<=operator>operator>=operator<=> (removed in C++20)(C++20) | compares to another `unique_ptr` or with nullptr (function template)
make_sharedmake_shared_for_overwrite (C++20) | creates a shared pointer that manages a new object (function template)
allocate_sharedallocate_shared_for_overwrite (C++20) | creates a shared pointer that manages a new object allocated using an allocator (function template)
static_pointer_castdynamic_pointer_castconst_pointer_castreinterpret_pointer_cast (C++17) | applies `static_cast`, `dynamic_cast`, `const_cast`, or `reinterpret_cast` to the stored pointer (function template)
get_deleter | returns the deleter of specified type, if owned (function template)
operator==operator!=operator<operator<=operator>operator>=operator<=> (removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20) | compares with another `shared_ptr` or with nullptr (function template)
operator<<(std::shared_ptr) | outputs the value of the stored pointer to an output stream (function template)
operator<<(std::unique_ptr) (C++20) | outputs the value of the managed pointer to an output stream (function template)
std::swap(std::unique_ptr) (C++11) | specializes the std::swap algorithm (function template)
std::swap(std::shared_ptr) (C++11) | specializes the std::swap algorithm (function template)
std::swap(std::weak_ptr) (C++11) | specializes the std::swap algorithm (function template)
##### Smart pointer adaptor creation
out_ptr (C++23) | creates an `out_ptr_t` with an associated smart pointer and resetting arguments (function template)
inout_ptr (C++23) | creates an `inout_ptr_t` with an associated smart pointer and resetting arguments (function template)

std::atomic_is_lock_free(std::shared_ptr)std::atomic_load(std::shared_ptr)std::atomic_load_explicit(std::shared_ptr)std::atomic_store(std::shared_ptr)std::atomic_store_explicit(std::shared_ptr)std::atomic_exchange(std::shared_ptr)std::atomic_exchange_explicit(std::shared_ptr)std::atomic_compare_exchange_weak(std::shared_ptr)std::atomic_compare_exchange_strong(std::shared_ptr)std::atomic_compare_exchange_weak_explicit(std::shared_ptr)std::atomic_compare_exchange_strong_explicit(std::shared_ptr) (deprecated in C++20)(removed in C++26) | specializes atomic operations for `std::shared_ptr` (function template)

### Function-like entities
Defined in namespace `std::ranges`
##### Uninitialized storage
ranges::uninitialized_copy (C++20) | copies a range of objects to an uninitialized area of memory (algorithm function object)
ranges::uninitialized_copy_n (C++20) | copies a number of objects to an uninitialized area of memory (algorithm function object)
ranges::uninitialized_fill (C++20) | copies an object to an uninitialized area of memory, defined by a range (algorithm function object)
ranges::uninitialized_fill_n (C++20) | copies an object to an uninitialized area of memory, defined by a start and a count (algorithm function object)
ranges::uninitialized_move (C++20) | moves a range of objects to an uninitialized area of memory (algorithm function object)
ranges::uninitialized_move_n (C++20) | moves a number of objects to an uninitialized area of memory (algorithm function object)
ranges::uninitialized_default_construct (C++20) | constructs objects by default-initialization in an uninitialized area of memory, defined by a range (algorithm function object)
ranges::uninitialized_default_construct_n (C++20) | constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count (algorithm function object)
ranges::uninitialized_value_construct (C++20) | constructs objects by value-initialization in an uninitialized area of memory, defined by a range (algorithm function object)
ranges::uninitialized_value_construct_n (C++20) | constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (algorithm function object)
ranges::construct_at (C++20) | creates an object at a given address (algorithm function object)
ranges::destroy_at (C++20) | destroys an object at a given address (algorithm function object)
ranges::destroy (C++20) | destroys a range of objects (algorithm function object)
ranges::destroy_n (C++20) | destroys a number of objects in a range (algorithm function object)

### Synopsis

    #include <compare>

    namespace std {
      // pointer traits
      template<class Ptr> struct pointer_traits;
      template<class T> struct pointer_traits<T*>;

      // pointer conversion
      template<class T>
        constexpr T* to_address(T* p) noexcept;
      template<class Ptr>
        constexpr auto to_address(const Ptr& p) noexcept;

      // pointer alignment
      void* align(size_t alignment, size_t size, void*& ptr, size_t& space);
      template<size_t N, class T>
        constexpr T* assume_aligned(T* ptr);

      // explicit lifetime management
      template<class T>
        T* start_lifetime_as(void* p) noexcept;                                   // freestanding
      template<class T>
        const T* start_lifetime_as(const void* p) noexcept;                       // freestanding
      template<class T>
        volatile T* start_lifetime_as(volatile void* p) noexcept;                 // freestanding
      template<class T>
        const volatile T* start_lifetime_as(const volatile void* p) noexcept;     // freestanding
      template<class T>
        T* start_lifetime_as_array(void* p, size_t n) noexcept;                   // freestanding
      template<class T>
        const T* start_lifetime_as_array(const void* p, size_t n) noexcept;       // freestanding
      template<class T>
        volatile T* start_lifetime_as_array(volatile void* p, size_t n) noexcept; // freestanding
      template<class T>
        const volatile T* start_lifetime_as_array(const volatile void* p,         // freestanding
                                                  size_t n) noexcept;

      // allocator argument tag
      struct allocator_arg_t { explicit allocator_arg_t() = default; };
      inline constexpr allocator_arg_t allocator_arg{};

      // uses_allocator
      template<class T, class Alloc> struct uses_allocator;

      // uses_allocator
      template<class T, class Alloc>
        inline constexpr bool uses_allocator_v = uses_allocator<T, Alloc>::value;

      // uses-allocator construction
      template<class T, class Alloc, class... Args>
        constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                        Args&&... args) noexcept;
      template<class T, class Alloc, class Tuple1, class Tuple2>
        constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t,
                                                        Tuple1&& x, Tuple2&& y) noexcept;
      template<class T, class Alloc>
        constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept;
      template<class T, class Alloc, class U, class V>
        constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                        U&& u, V&& v) noexcept;
      template<class T, class Alloc, class U, class V>
        constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                        const pair<U, V>& pr) noexcept;
      template<class T, class Alloc, class U, class V>
        constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                        pair<U, V>&& pr) noexcept;
      template<class T, class Alloc, class... Args>
        constexpr T make_obj_using_allocator(const Alloc& alloc, Args&&... args);
      template<class T, class Alloc, class... Args>
        constexpr T* uninitialized_construct_using_allocator(T* p, const Alloc& alloc,
                                                             Args&&... args);

      // allocator traits
      template<class Alloc> struct allocator_traits;

      template<class Pointer, class SizeType = size_t>
      struct allocation_result {
        Pointer ptr;
        SizeType count;
      };

      // the default allocator
      template<class T> class allocator;
      template<class T, class U>
        constexpr bool operator==(const allocator<T>&, const allocator<U>&) noexcept;

      // addressof
      template<class T>
        constexpr T* addressof(T& r) noexcept;
      template<class T>
        const T* addressof(const T&&) = delete;

      // specialized algorithms
      // special memory concepts
      template<class I>
        concept no-throw-input-iterator = /* see description */;    // exposition only
      template<class I>
        concept no-throw-forward-iterator = /* see description */;  // exposition only
      template<class S, class I>
        concept no-throw-sentinel-for = /* see description */;      // exposition only
      template<class R>
        concept no-throw-input-range = /* see description */;       // exposition only
      template<class R>
        concept no-throw-forward-range = /* see description */;     // exposition only

      template<class NoThrowForwardIt>
        void uninitialized_default_construct(NoThrowForwardIt first,
                                             NoThrowForwardIt last);
      template<class ExecutionPolicy, class NoThrowForwardIt>
        void uninitialized_default_construct(ExecutionPolicy&& exec,
                                             NoThrowForwardIt first,
                                             NoThrowForwardIt last);
      template<class NoThrowForwardIt, class Size>
        NoThrowForwardIt
          uninitialized_default_construct_n(NoThrowForwardIt first, Size n);
      template<class ExecutionPolicy, class NoThrowForwardIt, class Size>
        NoThrowForwardIt
          uninitialized_default_construct_n(ExecutionPolicy&& exec,
                                            NoThrowForwardIt first, Size n);

      namespace ranges {
        template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S>
          requires default_initializable<iter_value_t<I>>
            I uninitialized_default_construct(I first, S last);
        template<no-throw-forward-range R>
          requires default_initializable<range_value_t<R>>
            borrowed_iterator_t<R> uninitialized_default_construct(R&& r);

        template<no-throw-forward-iterator I>
          requires default_initializable<iter_value_t<I>>
            I uninitialized_default_construct_n(I first, iter_difference_t<I> n);
      }

      template<class NoThrowForwardIterator>
        void uninitialized_value_construct(NoThrowForwardIterator first,
                                           NoThrowForwardIterator last);
      template<class ExecutionPolicy, class NoThrowForwardIt>
        void uninitialized_value_construct(ExecutionPolicy&& exec,
                                           NoThrowForwardIt first,
                                           NoThrowForwardIt last);
      template<class NoThrowForwardIt, class Size>
        NoThrowForwardIt
          uninitialized_value_construct_n(NoThrowForwardIt first, Size n);
      template<class ExecutionPolicy, class NoThrowForwardIt, class Size>
        NoThrowForwardIt
          uninitialized_value_construct_n(ExecutionPolicy&& exec,
                                          NoThrowForwardIt first, Size n);

      namespace ranges {
        template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S>
          requires default_initializable<iter_value_t<I>>
            I uninitialized_value_construct(I first, S last);
        template<no-throw-forward-range R>
          requires default_initializable<range_value_t<R>>
            borrowed_iterator_t<R> uninitialized_value_construct(R&& r);

        template<no-throw-forward-iterator I>
          requires default_initializable<iter_value_t<I>>
            I uninitialized_value_construct_n(I first, iter_difference_t<I> n);
      }

      template<class InputIt, class NoThrowForwardIt>
        NoThrowForwardIt uninitialized_copy(InputIt first, InputIt last,
                                            NoThrowForwardIt result);
      template<class ExecutionPolicy, class ForwardIt, class NoThrowForwardIt>
        NoThrowForwardIt uninitialized_copy(ExecutionPolicy&& exec,
                                            ForwardIt first, ForwardIt last,
                                            NoThrowForwardIt result);
      template<class InputIt, class Size, class NoThrowForwardIt>
        NoThrowForwardIt uninitialized_copy_n(InputIt first, Size n,
                                              NoThrowForwardIt result);
      template<class ExecutionPolicy, class ForwardIt, class Size,
               class NoThrowForwardIt>
        NoThrowForwardIt uninitialized_copy_n(ExecutionPolicy&& exec,
                                              ForwardIt first, Size n,
                                              NoThrowForwardIt result);

      namespace ranges {
        template<class I, class O>
          using uninitialized_copy_result = in_out_result<I, O>;
        template<input_iterator I, sentinel_for<I> S1,
                 no-throw-forward-iterator O, no-throw-sentinel-for<O> S2>
          requires constructible_from<iter_value_t<O>, iter_reference_t<I>>
            uninitialized_copy_result<I, O>
              uninitialized_copy(I ifirst, S1 ilast, O ofirst, S2 olast);
        template<input_range IR, no-throw-forward-range OR>
          requires constructible_from<range_value_t<OR>, range_reference_t<IR>>
            uninitialized_copy_result<borrowed_iterator_t<IR>, borrowed_iterator_t<OR>>
              uninitialized_copy(IR&& in_range, OR&& out_range);

        template<class I, class O>
          using uninitialized_copy_n_result = in_out_result<I, O>;
        template<input_iterator I, no-throw-forward-iterator O, no-throw-sentinel-for<O> S>
          requires constructible_from<iter_value_t<O>, iter_reference_t<I>>
            uninitialized_copy_n_result<I, O>
              uninitialized_copy_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast);
      }

      template<class InputIt, class NoThrowForwardIt>
        NoThrowForwardIt uninitialized_move(InputIt first, InputIt last,
                                            NoThrowForwardIt result);
      template<class ExecutionPolicy, class ForwardIt, class NoThrowForwardIt>
        NoThrowForwardIt uninitialized_move(ExecutionPolicy&& exec,
                                            ForwardIt first, ForwardIt last,
                                            NoThrowForwardIt result);
      template<class InputIt, class Size, class NoThrowForwardIt>
        pair<InputIt, NoThrowForwardIt>
          uninitialized_move_n(InputIt first, Size n, NoThrowForwardIt result);
      template<class ExecutionPolicy, class ForwardIt, class Size,
               class NoThrowForwardIt>
        pair<ForwardIt, NoThrowForwardIt>
          uninitialized_move_n(ExecutionPolicy&& exec,
                               ForwardIt first, Size n, NoThrowForwardIt result);

      namespace ranges {
        template<class I, class O>
          using uninitialized_move_result = in_out_result<I, O>;
        template<input_iterator I, sentinel_for<I> S1,
                 no-throw-forward-iterator O, no-throw-sentinel-for<O> S2>
          requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>>
            uninitialized_move_result<I, O>
              uninitialized_move(I ifirst, S1 ilast, O ofirst, S2 olast);
        template<input_range IR, no-throw-forward-range OR>
          requires constructible_from<range_value_t<OR>, range_rvalue_reference_t<IR>>
            uninitialized_move_result<borrowed_iterator_t<IR>, borrowed_iterator_t<OR>>
              uninitialized_move(IR&& in_range, OR&& out_range);

        template<class I, class O>
          using uninitialized_move_n_result = in_out_result<I, O>;
        template<input_iterator I,
                 no-throw-forward-iterator O, no-throw-sentinel-for<O> S>
          requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>>
            uninitialized_move_n_result<I, O>
              uninitialized_move_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast);
      }

      template<class NoThrowForwardIt, class T>
        void uninitialized_fill(NoThrowForwardIt first, NoThrowForwardIt last,
                                const T& x);
      template<class ExecutionPolicy, class NoThrowForwardIt, class T>
        void uninitialized_fill(ExecutionPolicy&& exec,
                                NoThrowForwardIt first, NoThrowForwardIt last,
                                const T& x);
      template<class NoThrowForwardIt, class Size, class T>
        NoThrowForwardIt
          uninitialized_fill_n(NoThrowForwardIt first, Size n, const T& x);
      template<class ExecutionPolicy, class NoThrowForwardIt, class Size, class T>
        NoThrowForwardIt
          uninitialized_fill_n(ExecutionPolicy&& exec,
                               NoThrowForwardIt first, Size n, const T& x);

      namespace ranges {
        template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S, class T>
          requires constructible_from<iter_value_t<I>, const T&>
            I uninitialized_fill(I first, S last, const T& x);
        template<no-throw-forward-range R, class T>
          requires constructible_from<range_value_t<R>, const T&>
            borrowed_iterator_t<R> uninitialized_fill(R&& r, const T& x);

        template<no-throw-forward-iterator I, class T>
          requires constructible_from<iter_value_t<I>, const T&>
            I uninitialized_fill_n(I first, iter_difference_t<I> n, const T& x);
      }

      // construct_at
      template<class T, class... Args>
        constexpr T* construct_at(T* location, Args&&... args);

      namespace ranges {
        template<class T, class... Args>
          constexpr T* construct_at(T* location, Args&&... args);
      }

      // destroy
      template<class T>
        constexpr void destroy_at(T* location);
      template<class NoThrowForwardIt>
        constexpr void destroy(NoThrowForwardIt first, NoThrowForwardIt last);
      template<class ExecutionPolicy, class NoThrowForwardIt>
        void destroy(ExecutionPolicy&& exec,
                     NoThrowForwardIt first, NoThrowForwardIt last);
      template<class NoThrowForwardIt, class Size>
        constexpr NoThrowForwardIt destroy_n(NoThrowForwardIt first, Size n);
      template<class ExecutionPolicy, class NoThrowForwardIt, class Size>
        NoThrowForwardIt destroy_n(ExecutionPolicy&& exec,
                                   NoThrowForwardIt first, Size n);

      namespace ranges {
        template<destructible T>
          constexpr void destroy_at(T* location) noexcept;

        template<no-throw-input-iterator I, no-throw-sentinel-for<I> S>
          requires destructible<iter_value_t<I>>
            constexpr I destroy(I first, S last) noexcept;
        template<no-throw-input-range R>
          requires destructible<range_value_t<R>>
            constexpr borrowed_iterator_t<R> destroy(R&& r) noexcept;

        template<no-throw-input-iterator I>
          requires destructible<iter_value_t<I>>
            constexpr I destroy_n(I first, iter_difference_t<I> n) noexcept;
      }

      // class template unique_ptr
      template<class T> struct default_delete;
      template<class T> struct default_delete<T[]>;
      template<class T, class D = default_delete<T>> class unique_ptr;
      template<class T, class D> class unique_ptr<T[], D>;

      template<class T, class... Args>
        unique_ptr<T> make_unique(Args&&... args);                                  // T is not array
      template<class T>
        unique_ptr<T> make_unique(size_t n);                                        // T is U[]
      template<class T, class... Args>
        /* unspecified */ make_unique(Args&&...) = delete;                          // T is U[N]

      template<class T>
        unique_ptr<T> make_unique_for_overwrite();                                  // T is not array
      template<class T>
        unique_ptr<T> make_unique_for_overwrite(size_t n);                          // T is U[]
      template<class T, class... Args>
        /* unspecified */ make_unique_for_overwrite(Args&&...) = delete;            // T is U[N]

      template<class T, class D>
        void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept;

      template<class T1, class D1, class T2, class D2>
        bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
      template<class T1, class D1, class T2, class D2>
        bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
      template<class T1, class D1, class T2, class D2>
        bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
      template<class T1, class D1, class T2, class D2>
        bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
      template<class T1, class D1, class T2, class D2>
        bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
      template<class T1, class D1, class T2, class D2>
        requires three_way_comparable_with<typename unique_ptr<T1, D1>::pointer,
                                           typename unique_ptr<T2, D2>::pointer>
        compare_three_way_result_t<typename unique_ptr<T1, D1>::pointer,
                                   typename unique_ptr<T2, D2>::pointer>
          operator<=>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

      template<class T, class D>
        bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;
      template<class T, class D>
        bool operator<(const unique_ptr<T, D>& x, nullptr_t);
      template<class T, class D>
        bool operator<(nullptr_t, const unique_ptr<T, D>& y);
      template<class T, class D>
        bool operator>(const unique_ptr<T, D>& x, nullptr_t);
      template<class T, class D>
        bool operator>(nullptr_t, const unique_ptr<T, D>& y);
      template<class T, class D>
        bool operator<=(const unique_ptr<T, D>& x, nullptr_t);
      template<class T, class D>
        bool operator<=(nullptr_t, const unique_ptr<T, D>& y);
      template<class T, class D>
        bool operator>=(const unique_ptr<T, D>& x, nullptr_t);
      template<class T, class D>
        bool operator>=(nullptr_t, const unique_ptr<T, D>& y);
      template<class T, class D>
        requires three_way_comparable<typename unique_ptr<T, D>::pointer>
        compare_three_way_result_t<typename unique_ptr<T, D>::pointer>
          operator<=>(const unique_ptr<T, D>& x, nullptr_t);

      template<class E, class T, class Y, class D>
        basic_ostream<E, T>& operator<<(basic_ostream<E, T>& os, const unique_ptr<Y, D>& p);

      // class bad_weak_ptr
      class bad_weak_ptr;

      // class template shared_ptr
      template<class T> class shared_ptr;

      // shared_ptr creation
      template<class T, class... Args>
        shared_ptr<T> make_shared(Args&&... args);                                  // T is not array
      template<class T, class A, class... Args>
        shared_ptr<T> allocate_shared(const A& a, Args&&... args);                  // T is not array

      template<class T>
        shared_ptr<T> make_shared(size_t N);                                        // T is U[]
      template<class T, class A>
        shared_ptr<T> allocate_shared(const A& a, size_t N);                        // T is U[]

      template<class T>
        shared_ptr<T> make_shared();                                                // T is U[N]
      template<class T, class A>
        shared_ptr<T> allocate_shared(const A& a);                                  // T is U[N]

      template<class T>
        shared_ptr<T> make_shared(size_t N, const remove_extent_t<T>& u);           // T is U[]
      template<class T, class A>
        shared_ptr<T> allocate_shared(const A& a, size_t N,
                                      const remove_extent_t<T>& u);                 // T is U[]

      template<class T>
        shared_ptr<T> make_shared(const remove_extent_t<T>& u);                     // T is U[N]
      template<class T, class A>
        shared_ptr<T> allocate_shared(const A& a, const remove_extent_t<T>& u);     // T is U[N]

      template<class T>
        shared_ptr<T> make_shared_for_overwrite();                                  // T is not U[]
      template<class T, class A>
        shared_ptr<T> allocate_shared_for_overwrite(const A& a);                    // T is not U[]

      template<class T>
        shared_ptr<T> make_shared_for_overwrite(size_t N);                          // T is U[]
      template<class T, class A>
        shared_ptr<T> allocate_shared_for_overwrite(const A& a, size_t N);          // T is U[]

      // shared_ptr comparisons
      template<class T, class U>
        bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
      template<class T, class U>
        strong_ordering operator<=>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;

      template<class T>
        bool operator==(const shared_ptr<T>& x, nullptr_t) noexcept;
      template<class T>
        strong_ordering operator<=>(const shared_ptr<T>& x, nullptr_t) noexcept;

      // shared_ptr specialized algorithms
      template<class T>
        void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept;

      // shared_ptr casts
      template<class T, class U>
        shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r) noexcept;
      template<class T, class U>
        shared_ptr<T> static_pointer_cast(shared_ptr<U>&& r) noexcept;
      template<class T, class U>
        shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;
      template<class T, class U>
        shared_ptr<T> dynamic_pointer_cast(shared_ptr<U>&& r) noexcept;
      template<class T, class U>
        shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept;
      template<class T, class U>
        shared_ptr<T> const_pointer_cast(shared_ptr<U>&& r) noexcept;
      template<class T, class U>
        shared_ptr<T> reinterpret_pointer_cast(const shared_ptr<U>& r) noexcept;
      template<class T, class U>
        shared_ptr<T> reinterpret_pointer_cast(shared_ptr<U>&& r) noexcept;

      // shared_ptr get_deleter
      template<class D, class T>
        D* get_deleter(const shared_ptr<T>& p) noexcept;

      // shared_ptr I/O
      template<class E, class T, class Y>
        basic_ostream<E, T>& operator<<(basic_ostream<E, T>& os, const shared_ptr<Y>& p);

      // class template weak_ptr
      template<class T> class weak_ptr;

      // weak_ptr specialized algorithms
      template<class T> void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;

      // class template owner_less
      template<class T = void> struct owner_less;

      // class template enable_shared_from_this
      template<class T> class enable_shared_from_this;

      // hash support
      template<class T> struct hash;
      template<class T, class D> struct hash<unique_ptr<T, D>>;
      template<class T> struct hash<shared_ptr<T>>;

      // atomic smart pointers
      template<class T> struct atomic;
      template<class T> struct atomic<shared_ptr<T>>;
      template<class T> struct atomic<weak_ptr<T>>;

      // class template out_ptr_t
      template<class Smart, class Pointer, class... Args>
        class out_ptr_t;

      // function template out_ptr
      template<class Pointer = void, class Smart, class... Args>
        auto out_ptr(Smart& s, Args&&... args);

      // class template inout_ptr_t
      template<class Smart, class Pointer, class... Args>
        class inout_ptr_t;

      // function template inout_ptr
      template<class Pointer = void, class Smart, class... Args>
        auto inout_ptr(Smart& s, Args&&... args);
    }

    // deprecated
    namespace std {
      template<class T>
        bool atomic_is_lock_free(const shared_ptr<T>* p);

      template<class T>
        shared_ptr<T> atomic_load(const shared_ptr<T>* p);
      template<class T>
        shared_ptr<T> atomic_load_explicit(const shared_ptr<T>* p, memory_order mo);

      template<class T>
        void atomic_store(shared_ptr<T>* p, shared_ptr<T> r);
      template<class T>
        void atomic_store_explicit(shared_ptr<T>* p, shared_ptr<T> r, memory_order mo);

      template<class T>
        shared_ptr<T> atomic_exchange(shared_ptr<T>* p, shared_ptr<T> r);
      template<class T>
        shared_ptr<T> atomic_exchange_explicit(shared_ptr<T>* p, shared_ptr<T> r, memory_order mo);

      template<class T>
        bool atomic_compare_exchange_weak(shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w);
      template<class T>
        bool atomic_compare_exchange_strong(shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w);
      template<class T>
        bool atomic_compare_exchange_weak_explicit(
          shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w,
          memory_order success, memory_order failure);
      template<class T>
        bool atomic_compare_exchange_strong_explicit(
          shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w,
          memory_order success, memory_order failure);
    }

#### Helper concepts

Note: These names are only for exposition, they are not part of the interface.

    template<class I>
    concept no-throw-input-iterator = // exposition only
      input_iterator<I> &&
      is_lvalue_reference_v<iter_reference_t<I>> &&
      same_as<remove_cvref_t<iter_reference_t<I>>, iter_value_t<I>>;

    template<class S, class I>
    concept no-throw-sentinel-for = sentinel_for<S, I>; // exposition only

    template<class R>
    concept no-throw-input-range = // exposition only
      ranges::range<R> &&
      no-throw-input-iterator<ranges::iterator_t<R>> &&
      no-throw-sentinel-for<ranges::sentinel_t<R>, ranges::iterator_t<R>>;

    template<class I>
    concept no-throw-forward-iterator = // exposition only
      no-throw-input-iterator<I> &&
      forward_iterator<I> &&
      no-throw-sentinel-for<I, I>;

    template<class R>
    concept no-throw-forward-range = // exposition only
      no-throw-input-range<R> &&
      no-throw-forward-iterator<ranges::iterator_t<R>>;

#### Class template std::pointer_traits

    namespace std {
      template<class Ptr> struct pointer_traits {
        using pointer         = Ptr;
        using element_type    = /* see description */;
        using difference_type = /* see description */;

        template<class U> using rebind = /* see description */;

        static pointer pointer_to(/* see description */ r);
      };

      template<class T> struct pointer_traits<T*> {
        using pointer         = T*;
        using element_type    = T;
        using difference_type = ptrdiff_t;

        template<class U> using rebind = U*;

        static constexpr pointer pointer_to(/* see description */ r) noexcept;
      };
    }

#### Class std::allocator_arg_t

    namespace std {
      struct allocator_arg_t { explicit allocator_arg_t() = default; };
      inline constexpr allocator_arg_t allocator_arg{};
    }

#### Class template std::allocator_traits

    namespace std {
      template<class Alloc> struct allocator_traits {
        using allocator_type     = Alloc;

        using value_type         = typename Alloc::value_type;

        using pointer            = /* see description */;
        using const_pointer      = /* see description */;
        using void_pointer       = /* see description */;
        using const_void_pointer = /* see description */;

        using difference_type    = /* see description */;
        using size_type          = /* see description */;

        using propagate_on_container_copy_assignment = /* see description */;
        using propagate_on_container_move_assignment = /* see description */;
        using propagate_on_container_swap            = /* see description */;
        using is_always_equal                        = /* see description */;

        template<class T> using rebind_alloc = /* see description */;
        template<class T> using rebind_traits = allocator_traits<rebind_alloc<T>>;

        static pointer allocate(Alloc& a, size_type n);
        static pointer allocate(Alloc& a, size_type n,
                                const_void_pointer hint);

        static constexpr allocation_result<pointer, size_type>
              allocate_at_least(Alloc& a, size_type n);

        static void deallocate(Alloc& a, pointer p, size_type n);

        template<class T, class... Args>
          static void construct(Alloc& a, T* p, Args&&... args);

        template<class T>
          static void destroy(Alloc& a, T* p);

        static size_type max_size(const Alloc& a) noexcept;

        static Alloc select_on_container_copy_construction(const Alloc& rhs);
      };
    }

#### Class template std::allocator

    namespace std {
      template<class T> class allocator {
       public:
        using value_type                             = T;
        using size_type                              = size_t;
        using difference_type                        = ptrdiff_t;
        using propagate_on_container_move_assignment = true_type;

        constexpr allocator() noexcept;
        constexpr allocator(const allocator&) noexcept;
        template<class U> constexpr allocator(const allocator<U>&) noexcept;
        constexpr ~allocator();
        constexpr allocator& operator=(const allocator&) = default;

        constexpr T* allocate(size_t n);
        constexpr allocation_result<T*> allocate_at_least(size_t n);
        constexpr void deallocate(T* p, size_t n);

        // deprecated
        using is_always_equal = true_type;
      };
    }

#### Class template std::default_delete

    namespace std {
      template<class T> struct default_delete {
        constexpr default_delete() noexcept = default;
        template<class U> default_delete(const default_delete<U>&) noexcept;
        void operator()(T*) const;
      };

      template<class T> struct default_delete<T[]> {
        constexpr default_delete() noexcept = default;
        template<class U> default_delete(const default_delete<U[]>&) noexcept;
        template<class U> void operator()(U* ptr) const;
      };
    }

#### Class template std::unique_ptr

    namespace std {
      template<class T, class D = default_delete<T>> class unique_ptr {
      public:
        using pointer      = /* see description */;
        using element_type = T;
        using deleter_type = D;

        // constructors
        constexpr unique_ptr() noexcept;
        explicit unique_ptr(pointer p) noexcept;
        unique_ptr(pointer p, /* see description */ d1) noexcept;
        unique_ptr(pointer p, /* see description */ d2) noexcept;
        unique_ptr(unique_ptr&& u) noexcept;
        constexpr unique_ptr(nullptr_t) noexcept;
        template<class U, class E>
          unique_ptr(unique_ptr<U, E>&& u) noexcept;

        // destructor
        ~unique_ptr();

        // assignment
        unique_ptr& operator=(unique_ptr&& u) noexcept;
        template<class U, class E>
          unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
        unique_ptr& operator=(nullptr_t) noexcept;

        // observers
        add_lvalue_reference_t<T> operator*() const noexcept(/* see description */);
        pointer operator->() const noexcept;
        pointer get() const noexcept;
        deleter_type& get_deleter() noexcept;
        const deleter_type& get_deleter() const noexcept;
        explicit operator bool() const noexcept;

        // modifiers
        pointer release() noexcept;
        void reset(pointer p = pointer()) noexcept;
        void swap(unique_ptr& u) noexcept;

        // disable copy from lvalue
        unique_ptr(const unique_ptr&) = delete;
        unique_ptr& operator=(const unique_ptr&) = delete;
      };

      template<class T, class D> class unique_ptr<T[], D> {
      public:
        using pointer      = /* see description */;
        using element_type = T;
        using deleter_type = D;

        // constructors
        constexpr unique_ptr() noexcept;
        template<class U> explicit unique_ptr(U p) noexcept;
        template<class U> unique_ptr(U p, /* see description */ d) noexcept;
        template<class U> unique_ptr(U p, /* see description */ d) noexcept;
        unique_ptr(unique_ptr&& u) noexcept;
        template<class U, class E>
          unique_ptr(unique_ptr<U, E>&& u) noexcept;
        constexpr unique_ptr(nullptr_t) noexcept;

        // destructor
        ~unique_ptr();

        // assignment
        unique_ptr& operator=(unique_ptr&& u) noexcept;
        template<class U, class E>
          unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
        unique_ptr& operator=(nullptr_t) noexcept;

        // observers
        T& operator[](size_t i) const;
        pointer get() const noexcept;
        deleter_type& get_deleter() noexcept;
        const deleter_type& get_deleter() const noexcept;
        explicit operator bool() const noexcept;

        // modifiers
        pointer release() noexcept;
        template<class U> void reset(U p) noexcept;
        void reset(nullptr_t = nullptr) noexcept;
        void swap(unique_ptr& u) noexcept;

        // disable copy from lvalue
        unique_ptr(const unique_ptr&) = delete;
        unique_ptr& operator=(const unique_ptr&) = delete;
      };
    }

#### Class std::bad_weak_ptr

    namespace std {
      class bad_weak_ptr : public exception {
      public:
        bad_weak_ptr() noexcept;
      };
    }

#### Class template std::shared_ptr

    namespace std {
      template<class T> class shared_ptr {
      public:
        using element_type = remove_extent_t<T>;
        using weak_type    = weak_ptr<T>;

        // constructors
        constexpr shared_ptr() noexcept;
        constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() { }
        template<class Y>
          explicit shared_ptr(Y* p);
        template<class Y, class D>
          shared_ptr(Y* p, D d);
        template<class Y, class D, class A>
          shared_ptr(Y* p, D d, A a);
        template<class D>
          shared_ptr(nullptr_t p, D d);
        template<class D, class A>
          shared_ptr(nullptr_t p, D d, A a);
        template<class Y>
          shared_ptr(const shared_ptr<Y>& r, element_type* p) noexcept;
        template<class Y>
          shared_ptr(shared_ptr<Y>&& r, element_type* p) noexcept;
        shared_ptr(const shared_ptr& r) noexcept;
        template<class Y>
          shared_ptr(const shared_ptr<Y>& r) noexcept;
        shared_ptr(shared_ptr&& r) noexcept;
        template<class Y>
          shared_ptr(shared_ptr<Y>&& r) noexcept;
        template<class Y>
          explicit shared_ptr(const weak_ptr<Y>& r);
        template<class Y, class D>
          shared_ptr(unique_ptr<Y, D>&& r);

        // destructor
        ~shared_ptr();

        // assignment
        shared_ptr& operator=(const shared_ptr& r) noexcept;
        template<class Y>
          shared_ptr& operator=(const shared_ptr<Y>& r) noexcept;
        shared_ptr& operator=(shared_ptr&& r) noexcept;
        template<class Y>
          shared_ptr& operator=(shared_ptr<Y>&& r) noexcept;
        template<class Y, class D>
          shared_ptr& operator=(unique_ptr<Y, D>&& r);

        // modifiers
        void swap(shared_ptr& r) noexcept;
        void reset() noexcept;
        template<class Y>
          void reset(Y* p);
        template<class Y, class D>
          void reset(Y* p, D d);
        template<class Y, class D, class A>
          void reset(Y* p, D d, A a);

        // observers
        element_type* get() const noexcept;
        T& operator*() const noexcept;
        T* operator->() const noexcept;
        element_type& operator[](ptrdiff_t i) const;
        long use_count() const noexcept;
        explicit operator bool() const noexcept;
        template<class U>
          bool owner_before(const shared_ptr<U>& b) const noexcept;
        template<class U>
          bool owner_before(const weak_ptr<U>& b) const noexcept;
      };

      template<class T>
        shared_ptr(weak_ptr<T>) -> shared_ptr<T>;
      template<class T, class D>
        shared_ptr(unique_ptr<T, D>) -> shared_ptr<T>;
    }

#### Class template std::weak_ptr

    namespace std {
      template<class T> class weak_ptr {
      public:
        using element_type = remove_extent_t<T>;

        // constructors
        constexpr weak_ptr() noexcept;
        template<class Y>
          weak_ptr(const shared_ptr<Y>& r) noexcept;
        weak_ptr(const weak_ptr& r) noexcept;
        template<class Y>
          weak_ptr(const weak_ptr<Y>& r) noexcept;
        weak_ptr(weak_ptr&& r) noexcept;
        template<class Y>
          weak_ptr(weak_ptr<Y>&& r) noexcept;

        // destructor
        ~weak_ptr();

        // assignment
        weak_ptr& operator=(const weak_ptr& r) noexcept;
        template<class Y>
          weak_ptr& operator=(const weak_ptr<Y>& r) noexcept;
        template<class Y>
          weak_ptr& operator=(const shared_ptr<Y>& r) noexcept;
        weak_ptr& operator=(weak_ptr&& r) noexcept;
        template<class Y>
          weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;

        // modifiers
        void swap(weak_ptr& r) noexcept;
        void reset() noexcept;

        // observers
        long use_count() const noexcept;
        bool expired() const noexcept;
        shared_ptr<T> lock() const noexcept;
        template<class U>
          bool owner_before(const shared_ptr<U>& b) const noexcept;
        template<class U>
          bool owner_before(const weak_ptr<U>& b) const noexcept;
      };

      template<class T>
        weak_ptr(shared_ptr<T>) -> weak_ptr<T>;
    }

#### Class template std::owner_less

    namespace std {
      template<class T = void> struct owner_less;

      template<class T> struct owner_less<shared_ptr<T>> {
        bool operator()(const shared_ptr<T>&, const shared_ptr<T>&) const noexcept;
        bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const noexcept;
        bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const noexcept;
      };

      template<class T> struct owner_less<weak_ptr<T>> {
        bool operator()(const weak_ptr<T>&, const weak_ptr<T>&) const noexcept;
        bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const noexcept;
        bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const noexcept;
      };

      template<> struct owner_less<void> {
        template<class T, class U>
          bool operator()(const shared_ptr<T>&, const shared_ptr<U>&) const noexcept;
        template<class T, class U>
          bool operator()(const shared_ptr<T>&, const weak_ptr<U>&) const noexcept;
        template<class T, class U>
          bool operator()(const weak_ptr<T>&, const shared_ptr<U>&) const noexcept;
        template<class T, class U>
          bool operator()(const weak_ptr<T>&, const weak_ptr<U>&) const noexcept;

        using is_transparent = /* unspecified */;
      };
    }

#### Class template std::enable_shared_from_this

    namespace std {
      template<class T> class enable_shared_from_this {
      protected:
        constexpr enable_shared_from_this() noexcept;
        enable_shared_from_this(const enable_shared_from_this&) noexcept;
        enable_shared_from_this& operator=(const enable_shared_from_this&) noexcept;
        ~enable_shared_from_this();

      public:
        shared_ptr<T> shared_from_this();
        shared_ptr<T const> shared_from_this() const;
        weak_ptr<T> weak_from_this() noexcept;
        weak_ptr<T const> weak_from_this() const noexcept;

      private:
        mutable weak_ptr<T> weak_this;  // exposition only
      };
    }

#### Class template std::atomic's specialization for std::shared_ptr

    namespace std {
      template<class T> struct atomic<shared_ptr<T>> {
        using value_type = shared_ptr<T>;
        static constexpr bool is_always_lock_free = /* implementation-defined */;

        bool is_lock_free() const noexcept;
        void store(shared_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;
        shared_ptr<T> load(memory_order order = memory_order::seq_cst) const noexcept;
        operator shared_ptr<T>() const noexcept;

        shared_ptr<T> exchange(shared_ptr<T> desired,
                               memory_order order = memory_order::seq_cst) noexcept;

        bool compare_exchange_weak(shared_ptr<T>& expected, shared_ptr<T> desired,
                                   memory_order success, memory_order failure) noexcept;
        bool compare_exchange_strong(shared_ptr<T>& expected, shared_ptr<T> desired,
                                     memory_order success, memory_order failure) noexcept;

        bool compare_exchange_weak(shared_ptr<T>& expected, shared_ptr<T> desired,
                                   memory_order order = memory_order::seq_cst) noexcept;
        bool compare_exchange_strong(shared_ptr<T>& expected, shared_ptr<T> desired,
                                     memory_order order = memory_order::seq_cst) noexcept;

        constexpr atomic() noexcept = default;
        atomic(shared_ptr<T> desired) noexcept;
        atomic(const atomic&) = delete;
        void operator=(const atomic&) = delete;
        void operator=(shared_ptr<T> desired) noexcept;

      private:
        shared_ptr<T> p;            // exposition only
      };
    }

#### Class template std::atomic's specialization for std::weak_ptr

    namespace std {
      template<class T> struct atomic<weak_ptr<T>> {
        using value_type = weak_ptr<T>;
        static constexpr bool is_always_lock_free = /* implementation-defined */;

        bool is_lock_free() const noexcept;
        void store(weak_ptr<T> desired, memory_order order = memory_order::seq_cst) noexcept;
        weak_ptr<T> load(memory_order order = memory_order::seq_cst) const noexcept;
        operator weak_ptr<T>() const noexcept;

        weak_ptr<T> exchange(weak_ptr<T> desired,
                             memory_order order = memory_order::seq_cst) noexcept;

        bool compare_exchange_weak(weak_ptr<T>& expected, weak_ptr<T> desired,
                                   memory_order success, memory_order failure) noexcept;
        bool compare_exchange_strong(weak_ptr<T>& expected, weak_ptr<T> desired,
                                     memory_order success, memory_order failure) noexcept;

        bool compare_exchange_weak(weak_ptr<T>& expected, weak_ptr<T> desired,
                                   memory_order order = memory_order::seq_cst) noexcept;
        bool compare_exchange_strong(weak_ptr<T>& expected, weak_ptr<T> desired,
                                     memory_order order = memory_order::seq_cst) noexcept;

        constexpr atomic() noexcept = default;
        atomic(weak_ptr<T> desired) noexcept;
        atomic(const atomic&) = delete;
        void operator=(const atomic&) = delete;
        void operator=(weak_ptr<T> desired) noexcept;

      private:
        weak_ptr<T> p;              // exposition only
      };
    }

#### Class template std::out_ptr_t

    namespace std {
      template<class Smart, class Pointer, class... Args>
      class out_ptr_t {
      public:
        explicit out_ptr_t(Smart&, Args...);
        out_ptr_t(const out_ptr_t&) = delete;

        ~out_ptr_t();

        operator Pointer*() const noexcept;
        operator void**() const noexcept;

      private:
        Smart& s;                   // exposition only
        tuple<Args...> a;           // exposition only
        Pointer p;                  // exposition only
      };
    }

#### Class template std::inout_ptr_t

    namespace std {
      template<class Smart, class Pointer, class... Args>
      class inout_ptr_t {
      public:
        explicit inout_ptr_t(Smart&, Args...);
        inout_ptr_t(const inout_ptr_t&) = delete;

        ~inout_ptr_t();

        operator Pointer*() const noexcept;
        operator void**() const noexcept;

      private:
        Smart& s;                   // exposition only
        tuple<Args...> a;           // exposition only
        Pointer p;                  // exposition only
      };
    }

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